The reaction dynamics for C–Br dissociation within BrH2C–C≡CH(ads) adsorbed on an Ag(111) surface has been investigated by combining density functional theory-based molecular dynamics simulations with short-time Fourier transform (STFT) analysis of the dipole moment autocorrelation function. Two possible reaction pathways for C–Br scission within BrH2C–C≡CH(ads) have been proposed on the basis of different initial structural models. Firstly, the initial perpendicular orientation of adsorbed BrH2C–C≡CH(ads) with a stronger C–Br bond will undergo dynamic rotation leading to the final parallel orientation of BrH2C–C≡CH(ads) to cause the C–Br scission, namely, an indirect dissociation pathway. Secondly, the initial parallel orientation of adsorbed BrH2C–C≡C(ads) with a weaker C–Br bond will directly cause the C–Br scission within BrH2C–C≡CH(ads), namely, a direct dissociation pathway. To further investigate the evolution of different vibrational modes of BrH2C–C≡CH(ads) along these two reaction pathways, the STFT analysis is performed to illustrate that the infrared (IR) active peaks of BrH2C–C≡CH(ads) such as vCH2 [2956 cm−1(s) and 3020 cm−1(as)], v≡CH (3320 cm−1) and vC≡C (2150 cm−1) gradually vanish as the rupture of C–Br bond occurs and then the resulting IR active peaks such as C=C=C (1812 cm−1), ω-CH2 (780 cm−1) and δ-CH (894 cm−1) appear due to the formation of H2C=C=CH(ads) which are in a good agreement with experimental reflection adsorption infrared spectrum (RAIRS) at temperatures of 110 and 200 K, respectively. Finally, the total energy profiles indicate that the reaction barriers for the scission of C–Br within BrH2C–C≡CH(ads) along both direct and indirect dissociation pathways are very close due to a similar rupture of C–Br bond leading to a similar transition state.